Energy-storage devices, such as, ultracapacitors (e.g., supercapacitors, electrochemical capacitors), have conventionally been designed for high rate-high cyclability energy storage, albeit at roughly a hundred times lower gravimetric and volumetric energy, as compared to lithium ion batteries. Some of the end-uses for ultracapacitors, for instance, include solar and wind capacity firming, grid power regulation and leveling, voltage control, windmill pitch control, heavy truck/bus transport start-stop assist, and peak power assist, industrial shut-down support, among others. One way to classify supercapacitor devices is by their primary charge storage mechanism. As one skilled in the art will understand, most commercial ultracapacitor devices are exclusively electrical double layer capacitance (EDLC) devices which, for instance, store energy through reversible accumulation of charged ions in a double layer at an electrode's surface. While research is ongoing in alternative electrolytes, such as, ionic liquids (ILs), most commercial devices operate using a salt dissolved in an organic electrolyte that is designed to minimize parasitic side reactions at high voltages, for instance considering the Maxwell K2 Cell offering. It is recognized however that an aqueous electrolyte has substantial advantages in terms of safety, environmental friendliness and potential cost reduction. Akin to aqueous batteries, aqueous supercapacitors can have a major impact in the stationary energy storage arena where the intrinsic low cost and safety of water-based systems are a major premium.
As understood, pseudocapacitors are another class of ultracapacitor-like energy storage materials, with energy storage capability primarily originating from reversible faradaic reactions that occur at, and near the active material's (e.g., electrode's) surface. The use of pseudocapacitors on the positive electrode (also referred to as “hybrid devices”) does significantly boost the energy of aqueous devices, as the oxides or oxynitrides utilized therein have capacitance values as high as 1,000-2,000 F g−1. Moreover, a hybrid configuration (e.g., oxide on the positive electrode, carbon on the negative electrode), does somewhat extend the device voltage window by kinetically suppressing the decomposition of water above 1.2 V. Nanostructured oxides and nitrides, such as various forms of MnO2, Fe2O3, NiCo2O4, Nb2O5, MnMoO4/CoMoO4, Co3O4, CoMoO4 and VN are widely used for this application. As one skilled in the art will understand, the extent of the parallel EDLC contribution to energy storage in pseudocapacitor systems will depend on the surface area of the active (i.e., electrode) materials. Even the best performing nanostructured oxides, will typically possess surface areas that are at, or below 300 m2 g−1, which typically gives a relatively modest EDLC response. Oxide-based faradaic systems have seen less commercial activity, presumably due to a combination of increased electrode cost and the inherent propensity of most oxides to coarsen over time, due to voltage-induced dissolution or dissolution-reprecipitation. Unfortunately, oxides and oxynitrides are not fully stable during extended charging-discharging, leading to lifetimes of 10,000 cycles or less for even the state-of-the-art systems.
Thus, there remains a need for enhanced energy-storage devices, and in particular, a need for novel, organic EDLC device that utilizes an all-carbon symmetrical electrode configuration in an aqueous electrolyte for enhanced performance efficiency of such devices.